Control unit for a program shutter

Abstract

A program shutter adapted to close shutter blades having the function of diaphragm blades by a cam member operated with an electric motor has improved control accuracy in high brightness. A blade opening and closing member for opening and closing the shutter blades is urged to open the blades and driven to close the blades by the cam member operated by the motor. Brightness of an object to be photographed is measured by a brightness measuring portion and an area of a plurality of areas of brightness in which the measured brightness is contained is decided by a brightness decision portion. An exposure control circuit starts measurement of the exposure time when the shutter blades are operated to a predetermined trigger point. The cam member is previously moved to the limitation that movement of a blade driving member is not prevented in accordance with the brightness area in which the current brightness is contained before the exposure control circuit generates the exposure end signal. Thus, a closing delay time at time that the blade opening and closing member is moved to close the shutter by the cam member in response to the exposure end signal is shortened. Consequently, the trigger point for starting the exposure control circuit can be established in a position in which moving operation of the shutter blades just before the shutter blades form a pin hole is stable and exposure accuracy is improved.

What is claimed is: 
     
       1. A control unit for a program shutter, comprising: a plurality of shutter blades;   a blade opening and closing member connected with said plurality of shutter blades for opening and closing said shutter blades;   means for urging said blade opening and closing member to open said shutter blades;   an electric motor;   a cam member connected to said electric motor and engaged with said blade opening and closing member for rotation by said electric motor to selectively open and close said shutter blades;   brightness measuring means for measuring the brightness of an object to be photographed and producing an output value corresponding to the brightness;   a shutter blade position detecting means for generating a trigger signal when said shutter blades pass through a predetermined point during an opening operation of said shutter blades;   an exposure control means for generating an exposure end signal when a time corresponding to said output value of said brightness measuring means has elapsed, said time starting when said blade position detecting means generates said trigger signal;   brightness decision means for determining one of a plurality of areas of brightness of the object corresponding to said output value of said brightness measuring means; and   motor control means for controlling said electric motor such that when said electric motor rotates said cam member to open said shutter blades, said cam member is moved in the direction of closing of said shutter blades to a predetermined position corresponding to said area brightness determined by said brightness decision means before said exposure control means generates said exposure end signal while still allowing said blade opening and closing member to open said shutter blades to a desired aperture position, and for controlling said electric motor such that said cam member is rotated to close said shutter blades in response to said exposure end signal being generated by said exposure control means.   
     
     
       2. The control unit for a program shutter of claim 1, wherein said brightness decision means determines a said area of brightness for an object to be photographed with respect to predetermined brightness boundaries. 
     
     
       3. The control unit for a program shutter of claim 2, wherein said brightness boundaries separate areas of low, medium and high brightness, and wherein the time corresponding to said output value of said brightness measuring means is correspondingly changed according to said area of brightness that is determined. 
     
     
       4. The control unit for a program shutter of claim 3, wherein said motor comprises a stepping motor. 
     
     
       5. The control unit for a program shutter of claim 1, wherein said motor comprises a stepping motor. 
     
     
       6. The control unit for a program shutter of claim 1, wherein said blade opening and closing member comprises a cam follower engaged with said cam member, and said cam member has a plurality of cam surfaces thereon corresponding to closed and open positions of said shutter blades. 
     
     
       7. The control unit for a program shutter of claim 6, wherein said cam surface corresponding to said open position of said shutter blades has a plurality of predetermined positions thereon corresponding to respective said areas of brightness.